N. Chiodini

SnO2 nanocrystals in SiO2: A wide-band-gap quantum-dot system

Nanoclusters of crystalline SnO2 have been grown in glass, starting from tin-doped silica prepared by a sol–gel method. Based upon a particular choice of molecular precursors, nanocrystallites with a mean radius of about 1 nm and a quite narrow size dispersion were obtained, resulting in wide-band-gap (>4 eV) quantum dots (QDs). In this system, differently from other semiconductor-doped glasses, both the glassy host and the nanophase are oxides of IV-group elements. Owing to their thermochemical compatibility, the two phases give stable optical-grade glass ceramics with potential applications in photonics, opening up the field to technological employments of wide-band-gap QDs.

Ce3+-doped fibers for remote radiation dosimetry

A radioluminescent (RL) dosimetric system, based on a SiO2 optical fiber with the core doped by Ce3+ ions as luminescent activators has been investigated. Structural and optical properties of the luminescent fiber have been studied by Raman, refractive index, RL and scintillation time decay measurements, and compared to those obtained on bulk material. The RL response of a composite fiber made of a short portion of active Ce-doped fiber coupled to a long commercial one has been investigated by x-ray irradiation. A linear RL intensity response has been found in the dose rate interval 6×10−3–40mGy∕s together with a good radiation hardness, suggesting possible application in low-dose monitoring.

Erbium doped nanostructured tin-silicate glass-ceramic composites

Er-doped tin–silicate glass–ceramic composites were synthesized from Si, Er and Sn molecular precursors by following a sol–gel method. Optical spectroscopy and electron microscopy showed that the resulting material is composed of an amorphous silica network that encloses submicrometric SnO2 crystalline clusters. Analysis of the luminescence properties shows that Er ions are, at least partially, trapped in the crystalline phase. Raman spectra show that nanostructured tin–silicate composites act as low phonon energy hosts for rare earth ions and are thus suitable for photonic applications.

Ultraviolet photoluminescence of porous silica

Excitation pattern and decay kinetics of ultraviolet photoluminescence of porous silica are investigated between 4.5 and 10 eV by means of synchrotron radiation. Spectra are dominated by a 3.7 eV emission similar to the recently observed ultraviolet emission of oxidized porous Si and Si nanostructures. Emission intensity is found to be controlled by the material specific surface. Other emissions are observed at 2.9, 3.8, and 4.2 eV. All emissions show lifetimes of a few nanoseconds. Spectral and kinetic features are sensibly different than in glassy SiO2, suggesting a revision of previous assignments of ultraviolet emissions in oxidized porous Si and Si nanostructures.

High-efficiency SiO2:Ce3+ glass scintillators

We present the effect of a rapid thermal treatment (RTT) at high temperature (1800 °C) on the radio-luminescence properties of Ce-doped SiO2 glasses prepared by the sol–gel method and previously densified at 1050 °C. Cerium concentrations ranging from 0.05 up to 1 mol % were considered. We found that, for all concentrations, the RTT induces a strong increase of the Ce3+ radio-luminescence efficiency; the x-ray-induced luminescence intensity of the SiO2:0.1% Ce is about twice that of Bi3Ge4O12. The decay time of the scintillation response, evaluated as ≈50 ns, is not affected by RTT. Infrared absorption measurements indicate that the luminescence increase cannot be related to significant release of OH groups during RTT. The conversion of Ce4+ ions into Ce3+ ions can also be ruled out since an increase of about 20% of the intensity of the 4.8 eV optical absorption band related to Ce4+ was observed after RTT. The occurrence of dissolution of rare-earth aggregates is suggested.

Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices

The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices.

Photoluminescence of Sn-doped SiO2 excited by synchrotron radiation

Abstract Changes in the photoluminescence (PL) of amorphous SiO2 due to Sn-doping have been investigated by synchrotron radiation. Sn-doped SiO2 samples have been produced by a controlled sol–gel procedure as well as Ge-doped samples prepared for comparison. Detailed maps of the PL and PL excitation pattern have been obtained up to the band-to-band transition energy. The results confirm the analysis of Skuja as regards the emission at 3.1 and 4.2 eV excited at about 5 eV. At higher energies, our data show that the 3.1 and 4.2 eV PL bands have another excitation region with structures at 6.7, 7.2 and 8.0 eV. Lifetimes of about 10 ns for the 4.2 eV PL and 10 μs for the 3.1 eV PL are observed independently of the excitation energy. Data between 10 and 300 K are presented and compared with data from Ge-doped samples. The results show that high energy excitation of the 3.1 eV PL is not thermally activated, in contrast to the 4.9 eV excitation channel. Effects of the different spin–orbit coupling constants at Ge and Sn sites on PL intensity suggest that the high energy excitation channels arise from intra-center singlet-to-singlet transitions.

Ultraviolet free-exciton light emission in Er-passivated SnO2 nanocrystals in silica

SnO2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping to passivate the cluster boundaries. As a result, emission at 3.8eV from the decay of SnO2 free excitons is observed in nanostructured SnO2:SiO2, besides the extrinsic 2eV luminescence of defects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of the extrinsic emission competitive with the ultraviolet (UV) luminescence evidences the involvement of a phonon mode at 210cm−1 from a SnO-like phase. The feasibility of passivated wide-band-gap nanocrystals in silica gives interesting perspectives for UV-emitting optical devices.

Surface reactivity of SnO2 obtained by sol-gel type condensation: Interaction with inert, combustible gases, vapour-phase H2O and air, as revealed by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) studies have been carried out on lattice oxygen vacancies produced by the interaction of carbon monoxide with SnO 2 obtained by sol–gel type condensation. Under a 0.5% CO–argon reducing atmosphere the vacancies can transfer electrons to Sn 4+ producing Sn 2+ centres. In air the lattice defects interact with molecular oxygen in a manner which depends on whether the gas reducing treatment was performed under dry or moist conditions. Defects that undergo oxygen interaction at the SnO 2 surface, reduce O 2 to O 2 - or O 2- , depending on the temperature of the reaction with oxygen.

Cubic optical nonlinearity in nanostructured SnO2:SiO2

Nonlinear self-focusing and absorption effects were investigated in nanostructured SnO2:SiO2 by means of z-scan techniques. The results show electronic nonlinearity with the real part of the cubic susceptibility ranging between 0.5 and 2.3×10−20 m2/V2. Nonlinear dissipative effects appear to be caused by two-photon absorption involving localized states at the cluster interfaces, controlled by oxygen nonstoichiometry. As a result, synthesis can be optimized to give a wide-band-gap semiconductor-doped glass with good figure of merit for nonlinear directional couplers.